HOUSTON, July 5 (Reuters) - ConocoPhillips is moving to bring Venezuelan PDVSA’s U.S. refining unit Citgo Petroleum into its legal battles to enforce a $2 billion arbitration award over the South American country’s nationalization of its Venezuelan assets.

A federal judge in Houston on Thursday ruled the U.S. oil producer can depose Citgo as preparation for a court case against PDVSA over asset transfers in the Caribbean that Conoco claims were designed to frustrate its efforts to seize PDVSA assets in Curacao. (Reporting by Gary McWilliams Editing by James Dalgleish)